What would you do in this situation?

I walked past a new guy doing a very incredibly dangerous thing today. He had his hand within 1 inch of a 1/2 inch compression bit that was spinning at 15000 RPM. He was trying to adjust a coolant line. We pump compressed air though ours. Sometimes, if you are not careful, the air pressure blows the ends off of them, and can move your hand with it. The correct way to do this is to just pause the program, turn the spindle off, adjust the air line, and then continue, but, he was a new guy. and was doing something very capable of cutting his fingers off.

I froze. I did not want scream NO!! and startle him into turning around to me, and accidentaly shoving his hand directly into what I was trying to prevent. Obviously, more/better training is required, and he had no idea that what he was doing was stupidly dangerous. I spoke with him afterwards about it. What do you guys do when you walk up to a new guy doing a terrifingly dangerous thing in a machine shop?

Thanks.